The invention concerns a protective impedance for a mains voltage supplied electronic circuit with ohmic resistors applied as film on a substrate, at least two resistors forming a series connection.
A protective impedance of this kind is known from the international standard EN 60730-1, edition 1993-10(IEC 730-1), Annex H: "Automatic electrical controls for household and similar use". One of these protective impedances lies between one of the mains connections and the electronic circuit. A second protective impedance lies between the other mains connection and the electronic circuit. Each of the protective impedances comprises at least two single impedances connected in series, preferably ohmic resistors applied as oxide film, metal film, carbon film etc. on a substrate. The resistors are dimensioned so that the short-circuiting of one of the protective impedances, for example by an operator, and the short-circuiting of a resistor of the second protective impedance, for instance by a creepage current, will permit the flow of an AC of maximum 0.7 mA (peak value) or a DC of maximum 2 mA.
To meet these conditions, the individual resistors have relatively high resistance values. This again means that the operating current of the electronic circuit, which must flow through both protective impedances, is substantially lower than the mentioned maximum values, which often causes problems when designing the electronic circuit. Certainly an increase of the series-connected resistors of a protective impedance will cause an increase of the permitted operating current. However, this requires a correspondingly long substrate, for which space is often not available.